Mitigation of harmonic currents and conservation of power in non-linear load systems

Abstract

An AC power controller system applies three-phase AC operating power to an induction motor that drives a non-linear mechanical load. A primary low pass filter is connected in series between branch phase conductors and a power controller of the type that uses gate-controlled switching thyristors for controlling power to the motor. KVAR capacitors connected between the power controller and the induction motor phase windings form a secondary low pass filter across the controller output terminals. The primary and secondary low pass filters isolate the power controller and induction motor with respect to spurious noise and harmonics generated by local as well as remote sources, and also improve real power transfer efficiency from the power generating source to the induction motor by transforming the effective impedance of the power source and the induction motor load.

Description

REFERENCE TO RELATED ART[0001]This invention is related to the subject matter of U.S. Pat. No. 6,400,119 entitled “Energy Conserving Motor Controller,” which is assigned to the assignee of the present invention and incorporated herein by reference.BACKGROUND OF THE INVENTION[0002]1. Field of the Invention[0003]This invention is related generally to AC power distribution systems, and in particular to AC power controller systems that control the application of AC operating power to induction motors.[0004]2. Description of the Related Art[0005]Spurious noise signals, including harmonic currents, background noise and spike impulse noise are developed on AC power distribution lines. Such noise signals can originate from the power source, the distribution network, local and remote loads coupled to the network, lightning strikes and distribution equipment malfunction. The AC supply current delivered from a public utility is not a pure sine wave and contains harmonics that interfere with pr...

AI Technical Summary

Benefits of technology

"The improved power controller system described in this patent increases the efficiency and performance of AC induction motors by controlling the application of AC power to the stator windings. The system operates efficiently under both light and full-rated torque loading conditions, and also mitigates harmonic currents and load-induced harmonic currents. The system also includes a primary low pass filter and KVAR capacitors connected across the stator windings to improve low power factor caused by the high inductive impedance of the motor. These filters isolate the power controller and motor from spurious noise and harmonics generated by local and remote sources, and also improve real power transfer efficiency."

Problems solved by technology

Spurious noise signals, including harmonic currents, background noise and spike impulse noise are developed on AC power distribution lines.

Such noise signals can originate from the power source, the distribution network, local and remote loads coupled to the network, lightning strikes and distribution equipment malfunction.

The AC supply current delivered from a public utility is not a pure sine wave and contains harmonics that interfere with proper operation of connected equipment.

Additionally, noise and switching transients may be introduced from active loads.

This will introduce harmonics and high frequency noise on the power distribution conductors.

Such noise is not constant with respect to time, and it also varies from place to place in the power distribution network.

Each load can conduct a significant level of noise and harmonic currents back onto the power line, causing distortion of the power waveform.

Different loads and control devices produce different types and degrees of distortion that may interfere with the operation of the equipment and machines that are being supplied by the distribution network.

The amount of electric power used by machinery and the machinery itself can be affected by waveform distortions present in a power distribution system.

The injection of harmonic currents into the power distribution system can cause overheating of transformers and high neutral currents in three phase, grounded four wire systems.

As harmonic currents flow through the distribution system, voltage drops are produced for each individual harmonic, causing distortion of the applied voltage waveform, which is applied to all loads connected to the distribution bus.

These harmonic fluxes cause heat build-up and additional losses in the motor magnetic core, which reduce power transfer efficiency.

Induction motors can be damaged or degraded by harmonic current heating if the supply voltage is distorted.

The combination of these effects reduce power transfer efficiency and can cause motors to overheat and burn out.

Each harmonic of the fundamental frequency, depending on whether it is a positive, negative, or zero sequence, and its percentage of the fundamental, can have an adverse affect on motor performance and temperature rise, as well as increase the energy costs of electrical service that is charged by the utility service provider.

Most utilities charge a penalty to customers when the customer's total load power factor is low.

Harmonic current adds to the RMS value of the fundamental current supplied to the load, but does not provide any useful power.

An unacceptable load current phase angle difference can be expected because of the high inductive impedance presented by the stator windings of an induction motor.

When the induction motor is operating under discontinuous load conditions, or when the load is non-linear, high harmonic currents will result, degrading motor performance and reducing power factor.

Harmonic currents produced in the load circuit or that are conducted along the branch power distribution line from remote non-linear sources may find a resonance with the KVAR capacitors, and the resulting high current may cause the capacitors to fail.

These harmonic currents, when combined with the inductive reactance of the distribution network, can also cause premature motor failure due to excessive current flow, heat build-up and random breaker tripping.

Various problems arise in the operation of conventional controllers, particularly when controlling power applied to non-linear loads.

The constantly changing load between peak minimum and maximum values creates severe control difficulties for power factor control systems which must continuously adjust the power delivery to maintain optimum motor efficiency and economy.

Because of the fast on-off switching action (fast dv / dt) of the thyristors, high peak voltage and high switching frequency, the input current on the supply side of the power controller becomes distorted with high frequency switching transients, which cause an increase of harmonic components in the AC power delivered to the induction motor.

Moreover, spurious noise and harmonic currents from remote sources that are conducted down the branch distribution circuit can interfere with the proper switching operation of the controller itself, resulting in loss of power control.

These factors not only reduce the power factor of the branch load, but also interfere with motor operation and inject harmonic currents back through the power distribution branch and into the distribution network.

Moreover, controller-generated harmonic distortion increases the RMS value of the load current in the power distribution branch, on which the utility service fees are based, thus increasing the customer's energy costs.

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